The 3rd Generation Partnership Project (3GPP) LTE is an evolving high speed, high capacity standard for user equipment (UE) emerging as a globally accepted standard. LTE operates in Packet Switched (PS) domain. Currently, UEs equipped with LTE technology utilize LTE PS domain to handle data traffic, while the voice traffic is handled by legacy Circuit Switched (CS) networks (legacy Radio Access Technologies (RATs)) such as Global System for Mobile communications (GSM), Universal Mobile Terrestrial System (UMTS) and GSM EDGE Radio Access Network (GERAN) and so on. To support CS domain communication such as voice calls, the UE has to switch to CS supporting legacy RATs.
The Circuit Switched Fallback (CSFB) procedure standardized by 3GPP technical specification enables the UE s to perform this switching during Mobile Originated (MO) or Mobile Terminated Call (MT). The 3GPP standard defines Inter-RAT (I-RAT) for mobility (switching) between LTE and earlier 3GPP technologies such as UMTS and GERAN. To acquire a legacy RAT during I-RAT switching, the 3GPP standard provides a redirection based CSFB procedure.
In accordance with existing 3GPP standard for redirection based CSFB procedure, when the CS call is attempted while UE is in LTE, the UE starts an Extended Service Request (ESR) procedure. When the ESR is received by E-UTRAN Node B (eNodeB) of the LTE network, the eNodeB provides the UE with Radio Resource Control (RRC) connection release along with redirection request including the target RAT frequency on to which the UE can camp for the CS call.
In the existing system, during the CSFB procedure, the target Radio Network Controller (RNC) is not aware that a call establishment is due to CSFB. This indicates that the RNC cannot prioritize the RRC connection requests for CSFB over other types of RRC connections (example for data or signaling). Further, in a loaded RNC, the already delayed call setup for a CSFB call can be inadvertently delayed if RNC cannot prioritize the RRC connection establishment for the CSFB call.
If the RNC is aware that the call establishment is due to CSFB call, then it can prioritize the CSFB calls in case of overload conditions in the network. Further, the CSFB calls can be either a CS voice call or a CS data call. The CSFB is triggered due to either of voice calls or non-voice (data) calls. It is important for any network operator to give priority for CS voice calls ahead of CS data calls (for example USSD call, FAX call and so on).